Systems and methods for improved medical imaging

ABSTRACT

A radiation detector assembly is provided that includes a semiconductor detector, plural pixelated anodes, and at least one processor. The semiconductor detector has a surface. The plural pixelated anodes are configured to generate a primary signal responsive to reception of a photon by the pixelated anode and to generate at least one secondary signal responsive to an induced charge caused by reception of a photon by at least one surrounding anode. The at least one processor is configured to: acquire a primary signal from one of the anodes responsive to reception of a photon; acquire at least one secondary signal from at least one neighboring pixel of the one of the anodes, the at least one secondary signal defining a negative value; and determine an energy correction factor for the reception of the photon using the negative value of the at least one secondary signal.

RELATED APPLICATONS

The present application is a continuation-in-part of, and claims priority to, U.S. patent application Ser. No. 16/102,228, entitled “Systems and Methods for Determination of Depth of Interaction,” filed Aug. 13, 2018, the subject matter of which is hereby incorporated in its entirety.

BACKGROUND OF THE INVENTION

The subject matter disclosed herein relates generally to apparatus and methods for diagnostic medical imaging, such as Nuclear Medicine (NM) imaging.

In NM imaging, for example, systems with multiple detectors or detector heads may be used to image a subject, such as to scan a region of interest. For example, the detectors may be positioned adjacent the subject to acquire NM data, which is used to generate a three-dimensional (3D) image of the subject.

Imaging detectors may be used to detect reception of photons from an object (e.g., human patient that has been administered a radiotracer) by the imaging detector. The depth of interaction (DOI) or location along the thickness of a detector at which photons are detected may affect the strength of the signals generated by the detector responsive to the photons and be used to determine the number and location of detected events. Accordingly, the DOI may be used to correct detector signals to improve detector energy resolution and sensitivity. However, conventional approaches to determining DOI utilize signals from a cathode, requiring additional hardware and assembly complexity to utilize hardware to collect and process cathode signals. Also, cathodes tend to be relatively large and produce relatively noisy signals, reducing the accuracy and effectiveness of using signals from cathodes.

BRIEF DESCRIPTION OF THE INVENTION

In one embodiment, a radiation detector assembly is provided that includes a semiconductor detector, plural pixelated anodes, and at least one processor. The semiconductor detector has a surface. The plural pixelated anodes are disposed on the surface, with each pixelated anode configured to generate a primary signal responsive to reception of a photon by the pixelated anode and to generate at least one secondary signal responsive to an induced charge caused by reception of a photon by at least one surrounding anode. The at least one processor is operably coupled to the pixelated anodes, and configured to: acquire a primary signal from one of the anodes responsive to reception of a photon by the one of the anodes; acquire at least one secondary signal from at least one neighboring pixel of the one of the anodes responsive to an induced charge caused by the reception of the photon by the one of the anodes, the at least one secondary signal defining a negative value; and determine an energy correction factor for the reception of the photon by the one of the anodes using the negative value of the at least one secondary signal.

In another embodiment, a method of imaging using a semiconductor detector is provided. The semiconductor detector has a surface with plural pixelated anodes disposed thereon. Each pixelated anode is configured to generate a primary signal responsive to reception of a photon by the pixelated anode and to generate at least one secondary signal responsive to an induced charge caused by reception of a photon by at least one surrounding anode. The method includes acquiring a primary signal from one of the anodes responsive to reception of a photon by the one of the anodes. The method also includes acquiring at least one secondary signal from at least one neighboring pixel of the one of the anodes responsive to an induced charge caused by the reception of the photon by the one of the anodes. The at least one secondary signal defines a negative value. Further, the method includes determining an energy correction factor for the reception of the photon by the one of the anodes using the negative value of the at least one secondary signal.

In another embodiment, method is provided for providing a radiation detector assembly. The method includes providing a semiconductor detector having a surface with plural pixelated anodes disposed thereon. Each pixelated anode is configured to generate a primary signal responsive to reception of a photon by the pixelated anode and to generate at least one secondary signal responsive to an induced charge caused by reception of a photon by at least one adjacent anode. The method also includes operably coupling the pixelated anodes to at least one processor. Further, the method includes providing a calibrated radiation supply to the semiconductor detector. The pixelated anodes generate primary signals and secondary signals responsive to the calibrated radiation supply. Also, the method includes acquiring, with the at least one processor, the primary signals and the secondary signals from the pixelated anodes. The method also includes determining corresponding negative values of total induced charges for the secondary signals corresponding to different depths of absorption, and determining calibration information based on the negative values of the total induced corresponding to the different depths.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 depicts a representation of weighting potentials of a detector having a pixel biased by a voltage potential.

FIG. 2 depicts four events within the detector of FIG. 1.

FIG. 3 depicts corresponding induced charges for the four events of FIG. 2.

FIG. 4 depicts five groups of events under a primary or collecting pixel located at five different DOI's.

FIG. 5 depicts the resulting non-collected or secondary signals for the events located at Z₀ of FIG. 4.

FIG. 6 depicts the resulting non-collected or secondary signals for the events located at Z₁ of FIG. 4.

FIG. 7 depicts the resulting non-collected or secondary signals for the events located at Z₂ of FIG. 4.

FIG. 8 depicts the resulting non-collected or secondary signals for the events located at Z₃ of FIG. 4.

FIG. 9 depicts the resulting non-collected or secondary signals for the events located at Z₄ of FIG. 4.

FIG. 10 depicts a calibration system in accordance with various embodiments.

FIG. 11 provides a schematic view of a radiation detector assembly in accordance with various embodiments.

FIG. 12 provides a flowchart of a method in accordance with various embodiments.

FIG. 13 provides a flowchart of a method in accordance with various embodiments.

FIG. 14 provides a schematic view of an imaging system in accordance with various embodiments.

FIG. 15 provides a schematic view of an imaging system in accordance with various embodiments.

FIG. 16A schematically illustrates a radiation detector assembly in accordance with various embodiments.

FIG. 16B provides a graph showing signals produced by a primary pixel of the radiation detector assembly of FIG. 16A.

FIG. 16C provides a graph showing signals produced by a secondary pixel of the radiation detector assembly of FIG. 16A.

FIG. 17 provides a graph containing example empirical functions F(S) that may be derived from the radiation detector assembly of FIG. 16A by measuring and correlating the values of the primary signals E and the negative values S of the secondary induced signals.

FIG. 18 schematically illustrates multiple spectra produced by the radiation detector assembly of FIG. 16A.

FIG. 19 is a schematic illustration illustrating application of a correction factor K on the energies of the events to improve the energy-resolution and the sensitivity of the radiation detector assembly of FIG. 16A.

FIG. 20 provides a flowchart of a method in accordance with various embodiments.

FIG. 21 provides a flowchart of a method in accordance with various embodiments.

DETAILED DESCRIPTION OF THE INVENTION

The following detailed description of certain embodiments will be better understood when read in conjunction with the appended drawings. To the extent that the figures illustrate diagrams of the functional blocks of various embodiments, the functional blocks are not necessarily indicative of the division between hardware circuitry. For example, one or more of the functional blocks (e.g., processors or memories) may be implemented in a single piece of hardware (e.g., a general purpose signal processor or a block of random access memory, hard disk, or the like) or multiple pieces of hardware. Similarly, the programs may be stand alone programs, may be incorporated as subroutines in an operating system, may be functions in an installed software package, and the like. It should be understood that the various embodiments are not limited to the arrangements and instrumentality shown in the drawings.

As used herein, the terms “system,” “unit,” or “module” may include a hardware and/or software system that operates to perform one or more functions. For example, a module, unit, or system may include a computer processor, controller, or other logic-based device that performs operations based on instructions stored on a tangible and non-transitory computer readable storage medium, such as a computer memory. Alternatively, a module, unit, or system may include a hard-wired device that performs operations based on hard-wired logic of the device. Various modules or units shown in the attached figures may represent the hardware that operates based on software or hardwired instructions, the software that directs hardware to perform the operations, or a combination thereof.

“Systems,” “units,” or “modules” may include or represent hardware and associated instructions (e.g., software stored on a tangible and non-transitory computer readable storage medium, such as a computer hard drive, ROM, RAM, or the like) that perform one or more operations described herein. The hardware may include electronic circuits that include and/or are connected to one or more logic-based devices, such as microprocessors, processors, controllers, or the like. These devices may be off-the-shelf devices that are appropriately programmed or instructed to perform operations described herein from the instructions described above. Additionally or alternatively, one or more of these devices may be hard-wired with logic circuits to perform these operations.

As used herein, an element or step recited in the singular and preceded with the word “a” or “an” should be understood as not excluding plural of said elements or steps, unless such exclusion is explicitly stated. Furthermore, references to “one embodiment” of are not intended to be interpreted as excluding the existence of additional embodiments that also incorporate the recited features. Moreover, unless explicitly stated to the contrary, embodiments “comprising” or “having” an element or a plurality of elements having a particular property may include additional elements not having that property.

Various embodiments provide systems and methods for improving the sensitivity and/or energy resolution of image acquisition, for example in Nuclear Medicine (NM) imaging applications. In various embodiments, measurements of non-collected (or induced) adjacent transient signals are utilized to determine a depth of interaction (DOI) of corresponding events in a detector. It may be noted that the same measurements of the non-collected adjacent transient signals may also be used to determine corresponding sub-pixel locations for the events.

Generally, various embodiments provide methods and/or systems for measuring the negative value of induced signals and deriving or determining the corresponding DOI based on the negative value. For certain values of DOI (e.g., DOI's not located near an anode), all events having the same DOI produce about the same negative value for non-collected induced signals, regardless of their lateral position (with lateral position defined as x, y coordinates where the DOI is measured along a z axis). Accordingly, various embodiments use a measured value of induced signals (e.g., a measured negative value of non-collected induced signals, which may also be used for sub-pixel location determination) to derive or determine the DOI as well as to provide 3D positioning of events causing the non-collected induced signals.

A technical effect provided by various embodiments includes increased sensitivity and/or energy resolution of a detector system, such as a NM imaging detector system. A technical effect of various embodiments includes improved image quality. A technical effect of various embodiments includes reduced processing and/or hardware complexity associated with determining DOI via the elimination of use of signals from a cathode.

Before addressing specific aspects of particular embodiments, certain aspects of detector operation are discussed. FIG. 1 depicts a representation 10 of weighting potentials for a detector 11 having a pixelated anode 12 biased by a potential of 1 volt. Neighboring pixelated anodes 13 are not biased, or are at a ground potential of 0 volts. It may be noted that the depiction of a given pixelated anode at a voltage whereas neighboring pixelated anodes are not biased in connection with various examples herein is provided for clarity of illustration and ease of depiction; however, in practice each pixelated anode of a detector may be biased by a similar voltage. In the example depicted in FIG. 1, the cathode 14 is grounded at 0 volts. Solid curves 15 show lines of the electrical field, with dashed curves 16 showing lines of equipotential. The lines of equipotential are perpendicular to the lines of electrical field at the points where the lines cross.

The weighting potentials of FIG. 1 are depicted according to the Shockley-Ramo theorem. Under this theorem, the induced current produced by the weighting potential is described by i=qE*V=qE*V*cos(a), where i is the induced current, q is the electron charge, and E*V is the scalar product between the electrical field E of the weighting potential and the velocity V of the electron, and a is the angle between the vectors E and V.

FIGS. 2 and 3 depict the occurrence of events in various locations of the detector 11 and resulting induced charges. FIG. 2 depicts four events within the detector 11 of FIG. 1, and FIG. 3 depicts corresponding induced charges.

FIG. 2 depicts four events—event 21 starting at a depth of Z₀, event 22 starting at a depth of Z₁, event 23 starting a depth of Z₂, and event 24 starting at a depth of Z₃. Each of the events moves along a trajectory starting at X₁ and ending at the anode of the primary or collecting pixel 25 (the anode that collects the events). The non-collected induced charge on the adjacent pixel (or non-collecting pixel, in this case pixel 12, which is immediately neighboring the collecting pixel 25 in the illustrated example) is the integral over time (or over distance) of the current given by the relationship discussed above (i=qE*V=qE*V*cos(a)). E is the field of the weighting potential of the non-collecting adjacent pixel (pixel 12 in this example). Two ranges are shown over the depth D of the detector—a first range I, and a second range II.

In range I, the vector of the field has a component directed downward. Accordingly, the induced charge is positive over range I. Over range II, the vector of the field has a component that is directed upward. Accordingly, the induced charge is negative over range II. Event 21 at depth Z₀ starts at the cathode 14 and accordingly the related charge travels over the entire length of range I and range II. The other events start away from the cathode and accordingly the related charges do not travel over the entire depth of range I. Further, the event 24 at depth Z₃ starts within the boundary of range II, closer to the pixelated anodes than is the boundary of range II. Accordingly, the charge related to event 24 at depth Z₃ does not travel over the entire depth of range II.

FIG. 3 depicts resulting signals corresponding to the events of FIG. 2. Namely, signal 32 depicts the collecting or primary signal resulting in the collecting pixel 25. Signal 34 depicts the non-collecting signal of pixelated anode 12 resulting from event 21 starting at Z₀, signal 35 depicts the non-collecting signal of pixelated anode 12 resulting from event 22 starting at Z₁, signal 36 depicts the non-collecting signal of pixelated anode 12 resulting from event 23 starting at Z₂, and signal 37 depicts the non-collecting signal of pixelated anode 12 resulting from event 24 starting at Z₃.

As seen in FIG. 3, for the event 21 starting at Z₀ (the event occurring at the cathode 14 and traveling over the entire range of both range I and II), the total induced charge in ranges I and II is zero, as the positive induced charge in range I and the negative induced charge in range II are equal and cancel each other out over the full depth (e.g., at the anodes where Z=D, where D is the thickness of detector 11).

For the event 22 starting at Z₁ (which is away from the cathode), the total induced charge is negative, as the positive induced charge in range I is less than that from event 21, as the charge from event 22 does not traverse the entire depth of range I. Similarly, the total induced charge from event 23 is more negative than the total induced charge from event 22, and the total induced charge from event 24 is more negative than the total induced charge from event 23. This may be represented as [Q₀=0]>[Q₁<0]>[Q₂<0]>[Q₃<0], where Q₀ is the total induced charge for event 21 starting at Z₀, where Q₁ is the total induced charge for event 22 starting at Z₁, where Q₂ is the total induced charge for event 23 starting at Z₂, and where Q₃ is the total induced charge for event 24 starting at Z₃, Accordingly, as seen in FIG. 3, the closer an event is to the pixelated anodes (or farther from the cathode), the more negative the signal from a non-collected anode will tend to be.

FIG. 4 depicts five groups of events under a primary pixel 42 (the collecting pixel that generates a primary signal) located at five different DOI's: Z₀, Z₁, Z₂, Z₃, Z₄. Each group includes three events located at different X coordinates (namely X₁, X₂, and X₃) for a particular depth. The trajectories for each event moving toward the primary anode 42 (e.g., anode 25 of FIG. 2) are schematically shown in FIG. 4. These events move in the weighted potential and electrical field of the adjacent non-collecting pixel 44 (e.g., anode 12 of FIGS. 1 and 2) on which the non-collected charge is induced, resulting in a secondary signal generated by the adjacent, non-collecting pixel 44.

The resulting induced non-collected or secondary signals produced by each event are depicted in FIGS. 5-9. FIG. 5 includes a graph 50 that depicts the resulting non-collected or secondary signals for the events located at Z₀. As seen in FIG. 5, despite the fact that the group of events at Z₀ have different lateral locations X₁, X₂, and X₃, they all produce the same total non-collected induced charge signal that is equal to zero. It may be noted that all of the events at Z₀ start at the cathode. Curve 51 in FIG. 5 is the primary collected signal at primary anode 42, and is shown in FIG. 5 to help illustrate the differences between the primary signal and the secondary induced signal in terms of amplitude and shape.

FIG. 6 includes a graph 60 depicts the resulting non-collected or secondary signals for the events located at Z₁. As seen in FIG. 6, despite the fact that the group of events at Z₁ have different lateral locations X₁, X₂, and X₃, they all produce the same (or very nearly the same) total non-collected induced charge signal that is negative. It may be noted that all of the events at Z₁ start a small distance from the cathode and accordingly have a negative induced charge of a relatively small magnitude.

FIG. 7 includes a graph 70 depicts the resulting non-collected or secondary signals for the events located at Z₂. As seen in FIG. 7, despite the fact that the group of events at Z₂ have different lateral locations X₁, X₂, and X₃, they all produce about the same total non-collected induced charge signal that is negative (e.g., within range 72). It may be noted that all of the events at Z₂ start a larger distance from the cathode than for the event at Z₁ and accordingly have a relatively more negative induced charge of a relatively small magnitude.

FIG. 8 includes a graph 80 that depicts the resulting non-collected or secondary signals for the events located at Z₃. As seen in FIG. 8, despite the fact that the group of events at Z₃ have different lateral locations X₁, X₂, and X₃, they all produce about the same total non-collected induced charge signal that is negative (e.g., within range 82). It may be noted that all of the events at Z₃ start a larger distance from the cathode than for the event at Z₂ (and Z₁) and accordingly have a relatively more negative induced charge of a relatively small magnitude. It may be noted that the difference between the upper and lower value for the range 82 and the range 72 (see FIG. 7) are small enough to be ignored in various embodiments, so that the DOI is treated as independent of the lateral location.

FIG. 9 includes a graph 90 that depicts the resulting non-collected or secondary signals for the events located at Z₄. The negative charges for the events originating at a depth of Z₄ are substantially different from each other, due to the proximity of Z₄ to the anodes. Generally, in the illustrated example, as the depth of the event moves closer to the anode, the variability in the negative induced charge based on lateral position increases, with the variability becoming substantial only at depths very close to the anode.

As discussed above, except for events that start very close to a collecting anode, events produce a total non-collected induced charge in one or more adjacent anodes to the collecting anode that is correlated to the DOI of the event, substantially independent of lateral position. Accordingly, the total induced charge caused by an event on the adjacent pixelated anode (or pixelated anodes) that is zero or negative may be utilized for deriving or determining the DOI of the particular event. It may further be noted that due to the high absorption of the detector, very few events start close to the anodes, and accordingly such events may have a negligible effect on the use of negative induced charge to determine DOI. Various embodiments and methods disclosed herein accordingly determine a magnitude for a negative induced non-collected signal (also referred to herein as a secondary signal), and use the determined negative signal magnitude value to determine or derive DOI.

As discussed above, in various embodiments the DOI of an event may be derived from a correlation between the DOI of the event and the total non-collected induced charge on the adjacent pixel (or pixels) which is zero or negative, with the correlation between the DOI and the total non-collected induced charge being substantially independent of lateral position, such that lateral position may be disregarded in deriving the DOI. However, it may be noted that, for example, different photons may have different energies that can produce a different value for the total non-collected induced charge. Accordingly, in various embodiments, a detector system may be calibrated to account for different photon energies, for example to normalize the non-collected induced charge value to photon energy. Such a calibration process may be performed to provide calibration information that is used to determine DOI. The calibration information may be in the form of a look-up table, as one example, or in the form of a formula or mathematical expression based on a curve fitting, as another example.

FIG. 10 depicts a calibration system 92 in accordance with an embodiment. The depicted calibration system 92 is utilized to calibrate a detector 93 having a sidewall 94 that extends between an anode surface 95 and a cathode surface 96. The calibration system 92 includes a radiation source 97 and a pinhole collimator 98. The pinhole collimator 98 defines a scanning aperture that may be moved along the Z direction as seen in FIG. 10 to irradiate the sidewall 94 of the detector 93 at different DOI's (different Z coordinates). In this way, events with known DOI's and known photon energy are created having different lateral positions, with the lateral positions depending on the absorption statistic of the irradiation via the sidewall. By measuring resulting induced negative charges for different DOI's, the negative values of the total induced charge for non-collected adjacent signals may be used to create a look-up table or other relationship for deriving DOI from induced non-collected charge.

It may further be noted that, since the negative value of the induced charge also depends on the energy of the absorbed photon, the calibration may also account for photon energy. For example, the DOI may be calibrated based on a ratio between a negative value of the induced non-collected signal and the amplitude of the primary or collected signal. Such a ratio in various embodiments may be expressed as the following:

${DOI} \propto \frac{V_{\lbrack{{negative}\mspace{14mu}{value}\mspace{14mu}{of}\mspace{14mu}{the}\mspace{14mu}{induced}\mspace{14mu}{charge}}\rbrack}}{V_{\lbrack{{amplitude}\mspace{14mu}{of}\mspace{14mu}{the}\mspace{14mu}{primary}\mspace{14mu}{signal}}\rbrack}}$

Since the negative value of the induced signal is independent (or essentially or substantially independent as discussed herein) of the lateral position (or X,Y coordinates), all of the adjacent or neighboring pixels will produce similar negative signals. Accordingly, signal-to-noise ration may be improved by adding the negative signal from a number of adjacent or neighboring pixels. Such a ratio in various embodiments may be expressed as:

${DOI} \propto \frac{\sum\limits_{i = 1}^{i = N}\;{V\;\left\lbrack {{negative}\mspace{14mu}{value}\mspace{14mu}{of}\mspace{14mu}{the}\mspace{14mu}{induced}\mspace{14mu}{charge}} \right\rbrack}_{i}}{V_{\lbrack{{amplitude}\mspace{14mu}{of}\mspace{14mu}{the}\mspace{14mu}{primary}\mspace{14mu}{signal}}\rbrack}}$

It may be noted that the negative induced charge or signal of an adjacent pixel and the primary signal in various embodiments are measured after shapers that are configured to shape the received or acquired signals. In various embodiments both signals have a generally step-like shape and generally similar peaking and shaping times. Accordingly, the ratio between the signals may be about the same either after the shapers or immediately after amplifiers from which the shapers receive the signals.

FIG. 11 provides a schematic view of a radiation detector assembly 100 in accordance with various embodiments. As seen in FIG. 11, the radiation detector assembly 100 includes a semiconductor detector 110 and a processing unit 120. The semiconductor detector 110 has a surface 112 on which plural pixelated anodes 114 are disposed. In the depicted embodiment, a cathode 142 is disposed on a surface opposite the surface 112 on which the pixelated anodes 114 are disposed. For example, a single cathode may be deposited on one surface with the pixelated anodes disposed on an opposite surface. Generally, when radiation (e.g., one or more photons) impacts the pixelated anodes 114, the semiconductor detector 110 generates electrical signals corresponding to the radiation penetrating via the surface of cathode 142 and being absorbed in the volume of detector 110 under surface 112. In the illustrated embodiment, the pixelated anodes 114 are shown in a 5×5 array for a total of 25 pixelated anodes 114; however, it may be noted that other numbers or arrangements of pixelated anodes may be used in various embodiments. Each pixelated anode 114, for example, may have a surface area of 2.5 millimeters square; however, other sizes and/or shapes may be employed in various embodiments.

The semiconductor detector 110 in various embodiments may be constructed using different materials, such as semiconductor materials, including Cadmium Zinc Telluride (CdZnTe), often referred to as CZT, Cadmium Telluride (CdTe), and Silicon (Si), among others. The detector 110 may be configured for use with, for example, nuclear medicine (NM) imaging systems, positron emission tomography (PET) imaging systems, and/or single photon emission computed tomography (SPECT) imaging systems.

In the illustrated embodiment, each pixelated anode 114 generates different signals depending on the lateral location (e.g., in the X, Y directions) of where a photon is absorbed in the volume of detector 110 under the surface 112. For example, each pixelated anode 114 generates a primary or collected signal responsive to the absorption of a photon in the volume of detector 110 under the particular pixelated anode 114 through which the photon penetrates into the detector volume. The volumes of detector 110 under pixelated anodes 114 are defined as voxels (not shown). For each pixelated anode 114, detector 110 has the corresponding voxel. The absorption of a photon by a certain voxel corresponding to a particular pixelated anode 114 a also results in an induced charge that may be detected by pixels 114 b adjacent to or surrounding the particular pixelated anode 114 a that receives the photon. The charge detected by an adjacent or surrounding pixel may be referred to herein as a non-collected charge, and result in a non-collected or secondary signal. A primary signal may include information regarding photon energy (e.g., a distribution across a range of energy levels) as well as location information corresponding to the particular pixelated anode 114 at which a photon penetrates via the surface of cathode 142 and is absorbed in the corresponding voxel.

For example, in FIG. 1, a photon 116 is shown impacting the pixelated anode 114 a to be absorbed in the corresponding voxel. Accordingly, the pixelated anode 114 a generates a primary signal responsive to reception of the photon 116. As also seen in FIG. 1, pixelated anodes 114 b are adjacent to the pixelated anode 114 a. Pixelated anode 114 a has 8 adjacent pixelated anodes 114 b. When the pixelated anode 114 a is impacted by the photon 116, a charge is induced in and collected by the pixelated anode 114 a to produce the primary signal. One or more of the adjacent pixelated anodes 114 b generates a secondary signal responsive to the induced charge generated in and collected by the pixelated anode 114 a, which produces the primary signal. The secondary signal has an amplitude that is smaller than the primary signal. For any given photon, the corresponding primary signal (from the impacted pixel) and secondary signals (from one or more pixels adjacent to the impacted pixel) may be used to locate the reception point of a photon at a particular location within the pixel (e.g., to identify a particular sub-pixel location within the pixel).

As seen in FIG. 11, sidewalls 140 extend along a depth 150 in the Z direction between the surface 112 and the cathode 142. The location along the Z direction along the depth 150 of absorption where the photon 116 is absorbed is the DOI for the corresponding event. As discussed herein, the negative induced non-collected charge on one or more adjacent pixelated anodes 114 b is used in the illustrated embodiment to determine the DOI for the event corresponding to the impact of the photon 116.

Each pixelated anode 114 may have associated therewith one or more electronics channels configured to provide the primary and secondary signals to one or more aspects of the processing unit 120 in cooperation with the pixelated anodes. In some embodiments, all or a portion of each electronics channel may be disposed on the detector 110. Alternatively or additionally, all or a portion of each electronics channel may be housed externally to the detector 110, for example as part of the processing unit 120, which may be or include an Application Specific Integration Circuit (ASIC). The electronics channels may be configured to provide the primary and secondary signals to one or more aspects of the processing unit 120 while discarding other signals. For example, in some embodiments, each electronics channel includes a threshold discriminator. The threshold discriminator may allow signals exceeding a threshold level to be transmitted while preventing or inhibiting transmission of signals that do not exceed a threshold level. Generally, the threshold level is set low enough to reliably capture the secondary signals, while still being set high enough to exclude lower strength signals, for example due to noise. It may be noted that, because the secondary signals may be relatively low in strength, the electronics utilized are preferably low noise electronics to reduce or eliminate noise that is not eliminated by the threshold level. In some embodiments, each electronic channel includes a peak-and-hold unit to store electrical signal energy, and may also include a readout mechanism. For example, the electronic channel may include a request-acknowledge mechanism that allows the peak-and-hold energy and pixel location for each channel to be read out individually. Further, in some embodiments, the processing unit 120 or other processor may control the signal threshold level and the request-acknowledge mechanism.

In the illustrated embodiment, the processing unit 120 is operably coupled to the pixelated anodes 114, and is configured to acquire primary signals (for collected charges) and secondary signals (for non-collected charges). For example, the processing unit 120 in various embodiments acquires a primary signal from one of the anodes responsive to reception of a photon by the anode. For example, a primary signal may be acquired from pixelated anode 114 a responsive to reception of the photon 116. The processing unit 120 also acquires at least one secondary signal from at least one neighboring pixel (e.g., at least one adjacent anode 114 b) responsive to an induced charge caused by the reception of the photon. For example, a secondary signal may be acquired from one or more of the adjacent pixels 114 b responsive to reception of the photon 116. It may be noted that the secondary signal (or signals) and primary signal generated responsive to reception of the photon 116 may be associated with each other based on timing and location of detection of the corresponding charges.

The depicted processing unit 120 is also configured to determine a depth of interaction (DOI) in the semiconductor detector 110 for the reception of the photon using (e.g., based on) the at least one secondary signal. For example, a DOI along the depth 150 where the photon 116 is absorbed may be determined. In some embodiments, a total negative induced non-collected charge for the at least one secondary signal may be determined, and used to determine the DOI as discussed herein. In various embodiments, a lookup table or other correlation may be used to determine the DOI from a determined total negative induced non-collected charge for the at least one secondary signal. It may be noted that in various embodiments, the processing unit 120 determines the DOI using only signals generated based on information from the pixelated anodes 114, and without using any information from the cathode 142. Accordingly, construction and/or assembly of the detector assembly 100 may be avoid or eliminate any hardware or electrical connections that would otherwise be necessary for acquiring signals from the cathode 142 for use in determining DOI. Additionally, acquisitional and/or processing complexity or requirements may be further reduced by using the same information (primary and secondary signals) as discussed herein to determine both DOI and sub-pixel location.

The determined DOI may be utilized to improve image quality. For example, the determined DOI may be used to correct or adjust acquired imaging information. In some embodiments, the processing unit 120 is configured to adjust an energy level for an event corresponding to the reception of a photon by an anode based on the DOI. It may be noted that charge loss for a detected event depends on the distance of the absorption for the event from the anode. Accordingly, the DOI for a number of events may be used to adjust for charge loss to make the energy levels for the events more uniform and/or closer to a photopeak for accurate identification of events and accurate counting of events.

Alternatively or additionally, the processing unit 120 may be configured to reconstruct an image using the DOI. For example, the DOI of a number of events may be used directly by a reconstruction technique to utilize 3D positioning of events in the detector for reconstruction. As another example, the DOI may be used indirectly by a reconstruction technique via use of the DOI to correct energy levels, and then using the corrected energy levels for image reconstruction.

As discussed herein, calibration information is utilized in various embodiments. The processing unit 120 in various embodiments is configured to use calibration information (see, e.g., FIG. 10 and related discussion) to determine the DOI. The calibration may be in the form of a look up table or other relationship stored or otherwise associated with or accessible by the processing unit 120 (e.g., stored in memory 130). In some embodiments, the processing unit 120 is configured to determine the DOI using a calibration based on a ratio between a negative value of a single secondary signal and an amplitude of the primary signal. (See FIG. 10 and related discussion.) As another example, in some embodiments, the processing unit 120 is configured to determine the DOI using a calibration based on a ratio between a sum or combination of negative values for plural secondary signals (e.g., signals from a number of adjacent pixels 114 b) and an amplitude of the primary signal. (See FIG. 10 and related discussion.)

In various embodiments, the processing unit 120 may also be configured to determine a sub-pixel location (e.g., a lateral location) for events using the primary signal and at least one secondary signal in addition to determining the DOI. The sub-pixel location and the DOI may be determined using the same primary signal and at least one secondary signal, providing for efficient determination of both. For example, the depicted example processing unit 120 is configured to define sub-pixels for each pixelated anode. It may be noted that the sub-pixels in the illustrated embodiment (depicted as separated by dashed lines) are not physically separate, but instead are virtual entities defined by the processing unit 120. Generally, the use of increasing numbers of sub-pixels per pixel improves resolution while also increasing computational or processing requirements. The particular number of sub-pixels defined or employed in a given application may be selected based on a balance between improved resolution against increased processing requirements. In various embodiments, the use of virtual sub-pixels as discussed herein provides improved resolution while avoiding or reducing costs associated with increasingly larger number of increasingly smaller pixelated anodes.

In the illustrated embodiment, the pixelated anode 114 a is shown as divided into four sub-pixels, namely sub-pixel 150, sub-pixel 152, sub-pixel 154, and sub-pixel 156. While sub-pixels are shown in FIG. 11 for only pixelated anode 114 a for clarity and ease of illustration, it may be noted that the processing unit 120 in the illustrated embodiment also defines corresponding sub-pixels for each of the remaining pixelated anodes 114. As seen in FIG. 11, the photon 116 is impacting a portion of the pixelated anode 114 a defined by the virtual sub-pixel 150.

In the illustrated embodiment, the processing unit 120 acquires the primary signal for a given acquisition event (e.g., impact of a photon) from the pixelated anode 114 a, along with timing (e.g., timestamp) information corresponding to a time of generation of the primary signal and location information identifying the pixelated anode 114 a as the pixelated anode corresponding to the primary signal. For example, an acquisition event such as a photon impacting a pixelated anode 114 may result in a number of counts occurring across a range or spectrum of energies, with the primary signal including information describing the distribution of counts across the range or spectrum of energies. The processing unit 120 also acquires one or more secondary signals for the acquisition event from the pixelated anodes 114 b, along with timestamp information and location information for the secondary signal(s). The processing unit 120 then determines the location for the given acquisition event identifying the pixelated anode 114 a as the impacted pixelated anode 114 a, and then determining which of sub-pixels 150, 152, 154, 156 define the location of impact for the acquisition event. Using conventional methods, the location of sub-pixels 150, 152, 154, 156 may be derived based on the location (e.g., associated pixelated anode) and the relationships between the strengths of the primary signal in the associated pixelated anode 114 a and the secondary signal(s) in the adjacent pixelated anodes 114 b for the acquisition event. The processing unit 120 may use time stamp information as well as location information to associate the primary signal and secondary signals generated responsive to the given acquisition event with each other, and to discriminate the primary signal and secondary signals for the given acquisition event from signals for other acquisition events occurring during a collection or acquisition period using the time stamp and location information. Accordingly, the use of time stamp information helps allow for distinguishing between the primary signal and its corresponding secondary signals from random coincidence that may occur between primary signals of adjacent pixels, since the timestamps from the primary signal and its corresponding secondary signals are correlated for a particular acquisition event.

Additional discussion regarding virtual sub-pixels and the use of virtual sub-pixels, and the use of collected and non-collected charge signals may be found in U.S. patent application Ser. No. 14/724,022, entitled “Systems and Method for Charge-Sharing Identification and Correction Using a Single Pixel,” filed 28 May 2015 (“the 022 Application); U.S. patent application Ser. No. 15/280,640, entitled “Systems and Methods for Sub-Pixel Location Determination,” filed 29 Sep. 2016 (“the 640 Application”); and U.S. patent application Ser. No. 14/627,436, entitled “Systems and Methods for Improving Energy Resolution by Sub-Pixel Energy Calibration,” filed 20 Feb. 2015 (“the 436 Application). The subject matter of each of the 022 Application, the 640 Application, and the 436 Application are incorporated by reference in its entirety.

In various embodiments the processing unit 120 includes processing circuitry configured to perform one or more tasks, functions, or steps discussed herein. It may be noted that “processing unit” as used herein is not intended to necessarily be limited to a single processor or computer. For example, the processing unit 120 may include multiple processors, ASIC's, FPGA's, and/or computers, which may be integrated in a common housing or unit, or which may distributed among various units or housings. It may be noted that operations performed by the processing unit 120 (e.g., operations corresponding to process flows or methods discussed herein, or aspects thereof) may be sufficiently complex that the operations may not be performed by a human being within a reasonable time period. For example, the determination of values of collected and non-collected charges, and/or the determination of DOI's and/or sub-pixel locations based on the collected and/or non-collected charges within the time constraints associated with such signals may rely on or utilize computations that may not be completed by a person within a reasonable time period.

The depicted processing unit 120 includes a memory 130. The memory 130 may include one or more computer readable storage media. The memory 130, for example, may store mapping information describing the sub-pixel locations, acquired emission information, image data corresponding to images generated, results of intermediate processing steps, calibration parameters or calibration information (e.g., a lookup table correlating negative induced charge value to DOI), or the like. Further, the process flows and/or flowcharts discussed herein (or aspects thereof) may represent one or more sets of instructions that are stored in the memory 130 for direction of operations of the radiation detection assembly 100.

FIG. 12 provides a flowchart of a method 200 (e.g., for determining DOI), in accordance with various embodiments. The method 200, for example, may employ or be performed by structures or aspects of various embodiments (e.g., systems and/or methods and/or process flows) discussed herein. In various embodiments, certain steps may be omitted or added, certain steps may be combined, certain steps may be performed concurrently, certain steps may be split into multiple steps, certain steps may be performed in a different order, or certain steps or series of steps may be re-performed in an iterative fashion. In various embodiments, portions, aspects, and/or variations of the method 200 may be able to be used as one or more algorithms to direct hardware (e.g., one or more aspects of the processing unit 120) to perform one or more operations described herein.

At 202, primary signals and secondary signals are acquired corresponding to acquisition events (e.g., events corresponding to reception of photons). The primary and secondary signals are generated responsive to reception of photons by a semiconductor detector, and are received from pixelated anodes (e.g., anodes of a semiconductor device of an imaging system such as assembly 100). For example, a patient that has been administered at least one radiopharmaceutical may be placed within a field of view of one or more detectors, and radiation (e.g., photons) emitted from the patient may impact the pixelated anodes disposed on reception surfaces of the one or more detectors resulting in acquisition events (e.g., photon impacts). For a given photon impact in the depicted example embodiment, a primary signal (responsive to a collected charge) is generated by the impacted pixelated anode (or collecting anode), and one or more secondary signals (responsive to non-collected charge) are generated by pixelated anodes adjacent to the impacted pixelated anode (or non-collecting anodes).

At 204, a depth of interaction (DOI) in the semiconductor device is determined for the acquisition events resulting in the primary and secondary signals acquired at 202. In various embodiments, the DOI for a given event is determined using at least one secondary signal for that particular event. For example, as discussed herein, the DOI in various embodiments is determined based on a total negative induced non-collected charge value from one or more adjacent or non-collecting pixels (e.g., at least one adjacent pixelated anode). It may be noted that the DOI in various embodiments is determined without using any information (e.g., detected charge or corresponding signals) from a cathode of the detector assembly.

In various embodiments, the total negative non-collecting induced charge may be adjusted or corrected to account for variations in semiconductor construction and/or photon energies. For example, in the depicted embodiment, at 206, calibration information is used to determine the DOI. As discussed herein, in some embodiments, the DOI may be determined using a calibration based on a ratio between a negative value of a single secondary signal and an amplitude of the primary signal, and in some embodiments, the DOI may be determined using a calibration based on a ratio between a sum or combination of negative values for plural secondary signals and an amplitude of the primary signal. (See FIG. 10 and related discussion.)

At 208, a sub-pixel location is determined using the primary signal and the at least one secondary signal. For each event, a corresponding sub-pixel location may be determined. It may be noted that the same information (primary and secondary signals) used to determine DOI's for events may also be used to determine sub-pixel locations for those events.

At 210, an energy level for an event is adjusted based on the DOI. For example, as the energy detected may vary based on DOI, the DOI for each acquired event may be used to adjust the corresponding energy levels based on the corresponding DOI's to make the energy levels for a group of events more consistent and/or closer to a target or other predetermined energy level.

At 212, an image is reconstructed using the DOI. For example, corrected energy levels from 210 may be used in the reconstruction of an image. As another example, the DOIs for events may be used to determine 3D positioning information of those events within a detector, with the 3D positioning information used to reconstruct an image.

As discussed herein, a radiation detector system (e.g., a system configured to determine DOI using secondary signals corresponding to non-collected induced charges) may be calibrated. FIG. 13 provides a flowchart of a method 300 (e.g., for providing and calibrating a radiation detector assembly), in accordance with various embodiments. The method 300, for example, may employ or be performed by structures or aspects of various embodiments (e.g., systems and/or methods and/or process flows) discussed herein. In various embodiments, certain steps may be omitted or added, certain steps may be combined, certain steps may be performed concurrently, certain steps may be split into multiple steps, certain steps may be performed in a different order, or certain steps or series of steps may be re-performed in an iterative fashion. In various embodiments, portions, aspects, and/or variations of the method 300 may be able to be used as one or more algorithms to direct hardware (e.g., one or more aspects of the processing unit 120) to perform one or more operations described herein.

At 302, a semiconductor detector (e.g., semiconductor detector 110 of radiation imaging assembly 100) is provided. The semiconductor detector of the illustrated example has a surface with plural pixelated anodes disposed on the surface. Each pixelated anode is configured to generate a primary signal responsive to reception of a photon by the pixelated anode and to generate at least one secondary signal responsive to an induced charge caused by reception of a photon by at least one adjacent anode. At 304, the pixelated anodes are operably coupled to at least one processor (e.g., processing unit 120).

At 306, a calibrated radiation supply (e.g., having a known photon energy) is provided at different depths along a sidewall of the semiconductor detector. Responsive to reception of the calibrated radiation supply, the pixelated anodes generate primary and secondary signals. For example, the calibrated radiation supply may be passed through a pin-hole collimator to the sidewall of the semiconductor detector. The location of a given pin-hole (e.g., in a Z direction) through which radiation is passed may be used to determine the DOI at which the corresponding radiation is passed from the collimator and received by the semiconductor detector. At 308, the primary and secondary signals are acquired from the pixelated anodes by the at least one processor.

At 310, corresponding negative values of total induced charges for each of the different depths at which the radiation has been supplied are determined. At 312, calibration information (e.g., a look-up table or other correlating relationship between DOI's and negative induced non-collected charge values) is determined.

FIG. 14 is a schematic illustration of a NM imaging system 1000 having a plurality of imaging detector head assemblies mounted on a gantry (which may be mounted, for example, in rows, in an iris shape, or other configurations, such as a configuration in which the movable detector carriers 1016 are aligned radially toward the patient-body 1010). In particular, a plurality of imaging detectors 1002 are mounted to a gantry 1004. In the illustrated embodiment, the imaging detectors 1002 are configured as two separate detector arrays 1006 and 1008 coupled to the gantry 1004 above and below a subject 1010 (e.g., a patient), as viewed in FIG. 14. The detector arrays 1006 and 1008 may be coupled directly to the gantry 1004, or may be coupled via support members 1012 to the gantry 1004 to allow movement of the entire arrays 1006 and/or 1008 relative to the gantry 1004 (e.g., transverse translating movement in the left or right direction as viewed by arrow T in FIG. 14). Additionally, each of the imaging detectors 1002 includes a detector unit 1014, at least some of which are mounted to a movable detector carrier 1016 (e.g., a support arm or actuator that may be driven by a motor to cause movement thereof) that extends from the gantry 1004. In some embodiments, the detector carriers 1016 allow movement of the detector units 1014 towards and away from the subject 1010, such as linearly. Thus, in the illustrated embodiment the detector arrays 1006 and 1008 are mounted in parallel above and below the subject 1010 and allow linear movement of the detector units 1014 in one direction (indicated by the arrow L), illustrated as perpendicular to the support member 1012 (that are coupled generally horizontally on the gantry 1004). However, other configurations and orientations are possible as described herein. It should be noted that the movable detector carrier 1016 may be any type of support that allows movement of the detector units 1014 relative to the support member 1012 and/or gantry 1004, which in various embodiments allows the detector units 1014 to move linearly towards and away from the support member 1012.

Each of the imaging detectors 1002 in various embodiments is smaller than a conventional whole body or general purpose imaging detector. A conventional imaging detector may be large enough to image most or all of a width of a patient's body at one time and may have a diameter or a larger dimension of approximately 50 cm or more. In contrast, each of the imaging detectors 1002 may include one or more detector units 1014 coupled to a respective detector carrier 1016 and having dimensions of, for example, 4 cm to 20 cm and may be formed of Cadmium Zinc Telluride (CZT) tiles or modules. For example, each of the detector units 1014 may be 8×8 cm in size and be composed of a plurality of CZT pixelated modules (not shown). For example, each module may be 4×4 cm in size and have 16×16=256 pixels. In some embodiments, each detector unit 1014 includes a plurality of modules, such as an array of 1×7 modules. However, different configurations and array sizes are contemplated including, for example, detector units 1014 having multiple rows of modules.

It should be understood that the imaging detectors 1002 may be different sizes and/or shapes with respect to each other, such as square, rectangular, circular or other shape. An actual field of view (FOV) of each of the imaging detectors 1002 may be directly proportional to the size and shape of the respective imaging detector.

The gantry 1004 may be formed with an aperture 1018 (e.g., opening or bore) therethrough as illustrated. A patient table 1020, such as a patient bed, is configured with a support mechanism (not shown) to support and carry the subject 1010 in one or more of a plurality of viewing positions within the aperture 1018 and relative to the imaging detectors 1002. Alternatively, the gantry 1004 may comprise a plurality of gantry segments (not shown), each of which may independently move a support member 1012 or one or more of the imaging detectors 1002.

The gantry 1004 may also be configured in other shapes, such as a “C”, “H” and “L”, for example, and may be rotatable about the subject 1010. For example, the gantry 1004 may be formed as a closed ring or circle, or as an open arc or arch which allows the subject 1010 to be easily accessed while imaging and facilitates loading and unloading of the subject 1010, as well as reducing claustrophobia in some subjects 1010.

Additional imaging detectors (not shown) may be positioned to form rows of detector arrays or an arc or ring around the subject 1010. By positioning multiple imaging detectors 1002 at multiple positions with respect to the subject 1010, such as along an imaging axis (e.g., head to toe direction of the subject 1010) image data specific for a larger FOV may be acquired more quickly.

Each of the imaging detectors 1002 has a radiation detection face, which is directed towards the subject 1010 or a region of interest within the subject.

In various embodiments, multi-bore collimators may be constructed to be registered with pixels of the detector units 1014, which in one embodiment are CZT detectors. However, other materials may be used. Registered collimation may improve spatial resolution by forcing photons going through one bore to be collected primarily by one pixel. Additionally, registered collimation may improve sensitivity and energy response of pixelated detectors as detector area near the edges of a pixel or in-between two adjacent pixels may have reduced sensitivity or decreased energy resolution or other performance degradation. Having collimator septa directly above the edges of pixels reduces the chance of a photon impinging at these degraded-performance locations, without decreasing the overall probability of a photon passing through the collimator.

A controller unit 1030 may control the movement and positioning of the patient table 1020, imaging detectors 1002 (which may be configured as one or more arms), gantry 1004 and/or the collimators 1022 (that move with the imaging detectors 1002 in various embodiments, being coupled thereto). A range of motion before or during an acquisition, or between different image acquisitions, is set to maintain the actual FOV of each of the imaging detectors 1002 directed, for example, towards or “aimed at” a particular area or region of the subject 1010 or along the entire subject 1010. The motion may be a combined or complex motion in multiple directions simultaneously, concurrently, or sequentially as described in more detail herein.

The controller unit 1030 may have a gantry motor controller 1032, table controller 1034, detector controller 1036, pivot controller 1038, and collimator controller 1040. The controllers 1030, 1032, 1034, 1036, 1038, 1040 may be automatically commanded by a processing unit 1050, manually controlled by an operator, or a combination thereof. The gantry motor controller 1032 may move the imaging detectors 1002 with respect to the subject 1010, for example, individually, in segments or subsets, or simultaneously in a fixed relationship to one another. For example, in some embodiments, the gantry controller 1032 may cause the imaging detectors 1002 and/or support members 1012 to move relative to or rotate about the subject 1010, which may include motion of less than or up to 180 degrees (or more).

The table controller 1034 may move the patient table 1020 to position the subject 1010 relative to the imaging detectors 1002. The patient table 1020 may be moved in up-down directions, in-out directions, and right-left directions, for example. The detector controller 1036 may control movement of each of the imaging detectors 1002 to move together as a group or individually as described in more detail herein. The detector controller 1036 also may control movement of the imaging detectors 1002 in some embodiments to move closer to and farther from a surface of the subject 1010, such as by controlling translating movement of the detector carriers 1016 linearly towards or away from the subject 1010 (e.g., sliding or telescoping movement). Optionally, the detector controller 1036 may control movement of the detector carriers 1016 to allow movement of the detector array 1006 or 1008. For example, the detector controller 1036 may control lateral movement of the detector carriers 1016 illustrated by the T arrow (and shown as left and right as viewed in FIG. 14). In various embodiments, the detector controller 1036 may control the detector carriers 1016 or the support members 1012 to move in different lateral directions. Detector controller 1036 may control the swiveling motion of detectors 1002 together with their collimators 1022.

The pivot controller 1038 may control pivoting or rotating movement of the detector units 1014 at ends of the detector carriers 1016 and/or pivoting or rotating movement of the detector carrier 1016. For example, one or more of the detector units 1014 or detector carriers 1016 may be rotated about at least one axis to view the subject 1010 from a plurality of angular orientations to acquire, for example, 3D image data in a 3D SPECT or 3D imaging mode of operation. The collimator controller 1040 may adjust a position of an adjustable collimator, such as a collimator with adjustable strips (or vanes) or adjustable pinhole(s).

It should be noted that motion of one or more imaging detectors 1002 may be in directions other than strictly axially or radially, and motions in several motion directions may be used in various embodiment. Therefore, the term “motion controller” may be used to indicate a collective name for all motion controllers. It should be noted that the various controllers may be combined, for example, the detector controller 1036 and pivot controller 1038 may be combined to provide the different movements described herein.

Prior to acquiring an image of the subject 1010 or a portion of the subject 1010, the imaging detectors 1002, gantry 1004, patient table 1020 and/or collimators 1022 may be adjusted, such as to first or initial imaging positions, as well as subsequent imaging positions. The imaging detectors 1002 may each be positioned to image a portion of the subject 1010. Alternatively, for example in a case of a small size subject 1010, one or more of the imaging detectors 1002 may not be used to acquire data, such as the imaging detectors 1002 at ends of the detector arrays 1006 and 1008, which as illustrated in FIG. 14 are in a retracted position away from the subject 1010. Positioning may be accomplished manually by the operator and/or automatically, which may include using, for example, image information such as other images acquired before the current acquisition, such as by another imaging modality such as X-ray Computed Tomography (CT), MM, X-Ray, PET or ultrasound. In some embodiments, the additional information for positioning, such as the other images, may be acquired by the same system, such as in a hybrid system (e.g., a SPECT/CT system). Additionally, the detector units 1014 may be configured to acquire non-NM data, such as x-ray CT data. In some embodiments, a multi-modality imaging system may be provided, for example, to allow performing NM or SPECT imaging, as well as x-ray CT imaging, which may include a dual-modality or gantry design as described in more detail herein.

After the imaging detectors 1002, gantry 1004, patient table 1020, and/or collimators 1022 are positioned, one or more images, such as three-dimensional (3D) SPECT images are acquired using one or more of the imaging detectors 1002, which may include using a combined motion that reduces or minimizes spacing between detector units 1014. The image data acquired by each imaging detector 1002 may be combined and reconstructed into a composite image or 3D images in various embodiments.

In one embodiment, at least one of detector arrays 1006 and/or 1008, gantry 1004, patient table 1020, and/or collimators 1022 are moved after being initially positioned, which includes individual movement of one or more of the detector units 1014 (e.g., combined lateral and pivoting movement) together with the swiveling motion of detectors 1002. For example, at least one of detector arrays 1006 and/or 1008 may be moved laterally while pivoted. Thus, in various embodiments, a plurality of small sized detectors, such as the detector units 1014 may be used for 3D imaging, such as when moving or sweeping the detector units 1014 in combination with other movements.

In various embodiments, a data acquisition system (DAS) 1060 receives electrical signal data produced by the imaging detectors 1002 and converts this data into digital signals for subsequent processing. However, in various embodiments, digital signals are generated by the imaging detectors 1002. An image reconstruction device 1062 (which may be a processing device or computer) and a data storage device 1064 may be provided in addition to the processing unit 1050. It should be noted that one or more functions related to one or more of data acquisition, motion control, data processing and image reconstruction may be accomplished through hardware, software and/or by shared processing resources, which may be located within or near the imaging system 1000, or may be located remotely. Additionally, a user input device 1066 may be provided to receive user inputs (e.g., control commands), as well as a display 1068 for displaying images. DAS 1060 receives the acquired images from detectors 1002 together with the corresponding lateral, vertical, rotational and swiveling coordinates of gantry 1004, support members 1012, detector units 1014, detector carriers 1016, and detectors 1002 for accurate reconstruction of an image including 3D images and their slices.

It may be noted that the embodiment of FIG. 14 may be understood as a linear arrangement of detector heads (e.g., utilizing detector units arranged in a row and extending parallel to one another. In other embodiments, a radial design may be employed. Radial designs, for example, may provide additional advantages in terms of efficiently imaging smaller objects, such as limbs, heads, or infants. FIG. 15 provides a schematic view of a nuclear medicine (NM) multi-head imaging system 1100 in accordance with various embodiments. Generally, the imaging system 1100 is configured to acquire imaging information (e.g., photon counts) from an object to be imaged (e.g., a human patient) that has been administered a radiopharmaceutical. The depicted imaging system 1100 includes a gantry 1110 having a bore 1112 therethrough, plural radiation detector head assemblies 1115, and a processing unit 1120.

The gantry 1110 defines the bore 1112. The bore 1112 is configured to accept an object to be imaged (e.g., a human patient or portion thereof). As seen in FIG. 15, plural radiation detector head assemblies 1115 are mounted to the gantry 1110. In the illustrated embodiment, each radiation detector head assembly 1115 includes an arm 1114 and a head 1116. The arm 1114 is configured to articulate the head 1116 radially toward and/or away from a center of the bore 1112 (and/or in other directions), and the head 1116 includes at least one detector, with the head 1116 disposed at a radially inward end of the arm 1114 and configured to pivot to provide a range of positions from which imaging information is acquired.

The detector of the head 1116, for example, may be a semiconductor detector. For example, a semiconductor detector various embodiments may be constructed using different materials, such as semiconductor materials, including Cadmium Zinc Telluride (CdZnTe), often referred to as CZT, Cadmium Telluride (CdTe), and Silicon (Si), among others. The detector may be configured for use with, for example, nuclear medicine (NM) imaging systems, positron emission tomography (PET) imaging systems, and/or single photon emission computed tomography (SPECT) imaging systems.

In various embodiments, the detector may include an array of pixelated anodes, and may generate different signals depending on the location of where a photon is absorbed in the volume of the detector under a surface if the detector. The volumes of the detector under the pixelated anodes are defined as voxels. For each pixelated anode, the detector has a corresponding voxel. The absorption of photons by certain voxels corresponding to particular pixelated anodes results in charges generated that may be counted. The counts may be correlated to particular locations and used to reconstruct an image.

In various embodiments, each detector head assembly 1115 may define a corresponding view that is oriented toward the center of the bore 1112. Each detector head assembly 1115 in the illustrated embodiment is configured to acquire imaging information over a sweep range corresponding to the view of the given detector unit. Additional details regarding examples of systems with detector units disposed radially around a bore may be found in U.S. patent application Ser. No. 14/788,180, filed 30 Jun. 2015, entitled “Systems and Methods For Dynamic Scanning With Multi-Head Camera,” the subject matter of which is incorporated by reference in its entirety.

The processing unit 1120 includes memory 1122. The imaging system 1100 is shown as including a single processing unit 1120; however, the block for the processing unit 1120 may be understood as representing one or more processors that may be distributed or remote from each other. The depicted processing unit 1120 includes processing circuitry configured to perform one or more tasks, functions, or steps discussed herein. It may be noted that “processing unit” as used herein is not intended to necessarily be limited to a single processor or computer. For example, the processing unit 1120 may include multiple processors and/or computers, which may be integrated in a common housing or unit, or which may distributed among various units or housings.

Generally, various aspects (e.g., programmed modules) of the processing unit 1120 act individually or cooperatively with other aspects to perform one or more aspects of the methods, steps, or processes discussed herein. In the depicted embodiment, the memory 1122 includes a tangible, non-transitory computer readable medium having stored thereon instructions for performing one or more aspects of the methods, steps, or processes discussed herein.

It may be noted that the energy resolution of various radiation detectors, such as detectors made of CZT, and the low energy-tail of their spectrum are affected by the Depth of Interaction (DOI) at which the the photons of the detected radiation are absorbed in the detector. For example, generally, the deeper the photon is absorbed in the detector (e.g., a relatively large value for the DOI, or farther away from the cathode), the lower is the electrical charge (or magnitude of an electrical signal representing the photon energy) produced by the detector. Since the photons of the detected radiation are absorbed at a variety of DOI's, a corresponding variety of signals with different intensities (energies) will be produced.

It may be noted that the peak energy of the signals is produced when the photons are absorbed very close to the cathode of the detector. Photons that are absorbed relatively deeper but still close to the cathode produce signals with slightly lower energy than corresponding photons with the same energy that are absorbed closer to the cathode. These varying energies broaden the energy peak of the spectrum and can result in degraded energy-resolution. When photons with the same energy of the detected radiation are absorbed much deeper in the detector, the resulting energy signals may be significantly lower than the peak energy of the detector, and tend to increase the number of events that appear in the low-energy tail of the detector spectrum, resulting in a relatively higher low-energy tail, and resulting in detector sensitivity degradation. Generally, wide energy peaks and high low-energy tail are not desirable in the detector spectrum. These undesired phenomena may corrected using the conventionally known Hecht equation, which provides analytical mathematical relations between the DOI and the detector signal. However, such analytical approaches are correct only for detectors having uniform internal electrical fields and anodes having dimensions that are similar to the detector thickness (e.g., no small pixel effect). When the detector is affected by small-pixel effect, a correction may be applied to Hecht equation. However, when the internal electrical field of the detector is non-uniform, use of the Hecht equation may provide insufficient accuracy.

Accordingly, various embodiments provide systems and/or methods that empirically provide relationships (and/or utilize empirical relationships) between the DOI of a detector and the electrical signals that it produces, in a primary pixel, to allow energy-resolution and low-energy tail corrections based on the negative values of one or more secondary signals induced in pixels surrounding the primary pixel. Various embodiments provide systems and/or methods that empirically provide the relations (and/or utilize such relationships) between the negative induced signals developed in one or more pixels that are adjacent to the primary pixel that measures an event and the primary electrical signals to allow energy-resolution and low-energy tail corrections. Further, various embodiments provide systems and/or methods to correct the energy-resolution and low-energy tail of a detector spectrum using the negative values of the induced signals developed by one or more pixels that are adjacent to the primary pixel that produces the detector spectrum.

For example, FIG. 16A provides a schematic block view of a radiation detector assembly 1200 that develops and/or utilizes negative values of induced or secondary signals to correct energy levels and provide improved energy resolution and/or sensitivity. It may be noted that the radiation detector assembly 1200 depicted in FIG. 16 may be generally similar in various respects to the radiation detector assembly 100 discussed in connection with FIG. 11, and/or incorporate aspects of the radiation detector assembly 100. The depicted radiation detector assembly 1200 has a nonuniform internal electrical field.

It may be noted that for radiation detectors such as radiation detector assembly 1200 having nonuniform internal electrical fields (either with or without the small-pixel effect), the relationship between the DOI and the energy signal E produced by the detector, in a primary pixel, may be given by: E=f(DOI)  Eq(1)

It may be noted that the function f is not an analytical function (e.g., due to the nonuniform internal electrical field). As discussed above in connection with FIGS. 1-15, the negative value of the induced signal S, in a secondary pixel, depends on the DOI and may be given by another non-analytical function g as follows: S=g(DOI)  Eq (2)

Accordingly, the DOI may be expressed as follows: DOI=g ⁻¹(S)  Eq(3)

Where g⁻¹ is the inverse function of g. Accordingly, the energy signal E may also expressed as: E=f(DOI)=f(g ⁻¹(S))  Eq(4)

Put another way: E=F(S)  Eq(5)

Where: F(S)=f(g ⁻¹(S))  Eq(6)

It may be noted that F(S) is not an analytical function. Various techniques to derive F(S) empirically for correcting the energy-resolution and the low-energy tail are discussed below.

FIG. 16A schematically illustrates the radiation detector assembly 1200, and FIGS. 16B and 16C provide graphs 1242 and 1250, respectively, which show the signals output by the radiation detector assembly 1200 from its electronic channels 1209 and 1211 that are electrically connected to a primary pixel 1210 and a secondary pixel 1208, respectively. In the illustrated example, the secondary pixel 1208 is adjacent to the primary pixel 1210, with the primary pixel 1210 at a location corresponding to photon absorption in the radiation detector assembly 1200 as further discussed below.

The radiation detector assembly 1200 includes a semiconductor 1202 (e.g., a CZT semiconductor plate). The semiconductor 1202 has a surface 1203. The depicted radiation detector assembly 1200 also includes a monolithic cathode 1204, and pixilated anodes 1206, 1208, 1210. The anodes 1206, 1208, 1210 are electrically coupled to electronic channels 1209, 1211, and 1213, respectively, with the pixelated anodes 1206, 1208, 1210 disposed on the surface 1203, opposite the cathode 1204. The radiation detector assembly 1200 has a thickness 1230 that is equal to D. As seen in FIG. 16A, the depicted radiation detector assembly 1200 receives radiation including photons 1236 via the cathode 1204. The photons 1236 are absorbed in the radiation detector assembly 1200 at various DOI's measured along a Z coordinate illustrated by arrows 1232 in FIG. 16A. The lateral coordinate X is illustrated by arrow 1234. Pixels 1210 and 1208 define or correspond to voxels 1222 and 1220, respectively. The radiation detector assembly 1200 also includes a processing unit 1290 and memory 1292. The processing unit 1290, for example, receives signals from the electronic channels 1209, 1211, and 1213, and performs operations utilizing the signals as discussed herein. It may be noted that the processing unit 1290 may be generally similar to the processing unit 120, and/or incorporate one or more aspects of the processing unit 120. (For example, the processing unit 1290 may include processing circuitry configured to perform one or more tasks, functions, or steps discussed herein. Further, it may again be noted that “processing unit” as used herein is not intended to necessarily be limited to a single processor or computer. For example, the processing unit 1290 may include multiple processors, ASIC's, FPGA's, and/or computers, which may be integrated in a common housing or unit, or which may distributed among various units or housings.)

The processing unit 1290 in various embodiments is operably coupled to the pixelated anodes 1206, 1208, 1210, and is configured (e.g., programmed) to acquire a primary signal from one of the anodes (e.g., pixel 1210 in the illustrated example) responsive to reception of a photon by the anode, and acquire at least one secondary signal from at least one neighboring pixel (e.g., pixel 1208 in the illustrated example). As discussed herein, the at least one secondary signal defines a negative value. The depicted processing unit 1290 is further configured to determine an energy correction factor for the reception of the photon using the negative value of the at least one secondary signal. For example, as discussed in greater detail below, the processing unit 1290 may use a relationship of a value of the primary signal with the negative value of the at least one secondary signal to determine the energy correction factor. In some embodiments, the processing unit 1290 uses a plot of the negative value of the at least one secondary signal against the value of the primary signal. In some embodiments, the energy correction factor is defined by a ratio between a magnitude of the primary signal and a magnitude of the negative value. It may be noted that, as used herein, reference to the expression “negative value” may be used to describe the negative value of the at least one secondary signal.

Further, in various embodiments, the processing unit 1290 is configured to use the energy correction factor to adjust an energy level of detected events (e.g., multiplying a detected energy level by the correction factor to adjust the energy level toward or to a target level such as a maximum peak energy level). The processing unit 1290 may apply the energy correction factor to move at least one detected event from outside of an energy window used for counting (e.g., from a low energy tail located outside of the energy window) to inside the energy window used for counting, to increase the number of counts without requiring additional scanning time. In various embodiments, the processing unit 1290 is configured to organize detected events into groups based on magnitudes of corresponding negative values, and to apply corresponding energy correction factors to the groups. For example, the processing unit 1290 may be configured to define sub-spectra based on magnitudes of corresponding negative values for detected events, and align peaks of the sub-spectra using corresponding energy correction factors (e.g., a different energy correction factor for each group based on each individual group's negative value of the secondary signal). Additional details regarding the determination and/or application of energy correction factors are discussed below.

Generally, the photons 1236 of the illustrated example are absorbed in radiation detector assembly 1200 at various DOI's for corresponding detection events. Each of the events occurs in the voxel 1222 defined by or corresponding to the primary pixel 1210. For example, DOI₀ for event 1228 is located at Z=0 and is marked as Z₀, DOI₁ for event 1226 is located at Z=Z₁ and is marked as Z₁, and DOI₂ for event 1224 is located at Z=Z₂ and is marked as Z₂. Photons 1236 absorbed at respective depth DOI₀, DOI₁ and DOI₂ produce corresponding events 1228, 1226, and 1224 shown on lines 1218, 1216, and 1214, respectively. The signals produced by events 1228, 1226 and 1224, in electronic channel 1209 by primary pixel 1210, and in electronic channel 1211 by secondary pixel 1208 are shown by graph 1242 (FIG. 16B) and graph 1250 (FIG. 16C).

As seen in FIG. 16B, graph 1242 schematically shows the primary signals of the charge Q (proportional to the energy signal E) induced and collected by the primary pixel 1210 as a function of the coordinate Z along which the charges of events 1228, 1226 and 1224 drift in voxel 1222 toward the anode of primary pixel 1210 for collection by the primary pixel 1210. Graph 1242 shows the induced and collected charge on the primary anode or primary pixel 1210 produced by events 1228, 1226, and 1224 absorbed, respectively, at 1218 (Z=0), 1216 (Z=Z₁), and 1214 (Z=Z₂). The charges produced by events 1228, 1226, and 1224 created at (Z=Z₀), (Z=Z₁), and (Z=Z₂) as they drift toward the primary anode or primary pixel 1210 along coordinate Z are shown by curves 1244, 1246, and 1248 and marked by Q(Z=Z₀, Z), Q(Z=Z₁, Z), and Q(Z=Z₂, Z). The charges produced by events 1228, 1226, and 1224 when their charges arrive to the primary pixel 1210 (Z=D) are marked by Q (Z=Z₀, D), Q (Z=Z₁, D) and Q (Z=Z₂, D). As seen in FIG. 16B, the primary signal (Q or E) depends on the DOI, such that it is different for each different DOI. The maximum signal Q or E occurs when the DOI=0. In such a case, the energy of the primary signal measured by pixel 1210 is maximal and is equal to the peak-energy E_(P) in the spectrum of pixel 1210 of the radiation detector assembly 1200. It may be noted that where reference is made to the primary induced and collected charge Q, it may be replaced by the primary energy signal E since Q and E are proportional to each other. Accordingly, it may be noted that references to charge Q are also generally similar to references to energy E, and vice versa.

As seen in FIG. 16C, graph 1250 schematically shows the secondary induced signals of the charge Q (proportional to the energy of the secondary induced signal E or S), induced on the secondary anode or secondary pixel 1208 that is adjacent to primary pixel 1210, as a function of the coordinate Z, along which the charges of events 1228, 1226 and 1224 in primary voxel 1222 drift toward the primary pixel 1210 for collection. Graph 1250 shows the induced charge on the secondary pixel 1208 produced by events 1228, 1226, and 1224 generated at 1218 (Z=0), 1216 (Z=Z₁) and 1214 (Z=Z₂), respectively. The non-collected induced charges produced by events 1228, 1226 and 1224, on the secondary pixel 1208, as the events are generated at (Z=Z₀), (Z=Z₁) and (Z=Z₂) and drift toward the anode of pixel 1210, along coordinate Z, are shown by curves 1252, 1254, and 1256, and marked by Q(Z=Z₀, Z), Q(Z=Z₁, Z) and Q(Z=Z₂, Z). The negative charges produced by events 1228, 1226 and 1224, on the secondary anode of secondary pixel 1208, when their charges arrive at the primary pixel 1210 (Z=D) and are collected at the primary pixel 1210, are marked by Q (Z=Z₀, D), Q (Z=Z₁, D) and Q (Z=Z₂, D). The negative charges on the secondary anode or pixel produced when charges arrive at the primary pixel provide a negative value of secondary signal that may be utilized to determine an energy correction factor as discussed herein. For example, the value (or the magnitude) of the value of Q for a given curve at the location on the horizontal axis where Z=D may be used as the negative value as discussed herein. As seen in FIG. 16C, it may be noted that the secondary signal (charge Q, energy E or secondary induced signal S) depends on the DOI, such that it is different for each different DOI. The negative value S of the signal Q or E is minimum and is equal to zero for events occurring at Z=0. In such a case, the energy of the primary signal measured by pixel 1210 is maximal and is equal to the peak-energy E_(P) in the spectrum of pixel 1210 of the radiation detector assembly 1200. The deeper the DOI of a given event, the higher in magnitude the negative value S of the induced signal for the given event will be. It should be noted that, where reference is made to the secondary induced charge Q, it can also correspond or refer to the secondary energy signal E or S since Q, E, and S are proportional to each other. Accordingly, it may be noted that references to charge Q are also generally similar to references energy E or S and vice versa.

From graphs 1242 (FIG. 16B) and 1250 (FIG. 16C), it can be seen that for each DOI there is a corresponding value of the primary signal Q or E (graph 1242) and also a corresponding negative value of the induced secondary signal S, Q or E (graph 1250). Accordingly, the values of the primary signals E (graph 1242) are correlated with the negative values of the secondary induced signals S (graph 1250). In various embodiments, the relationship between the value of the primary signal and the negative value of the secondary signal (or signals) may be used to determine an energy correction factor and/or to adjust an energy level of detected events to improve sensitivity and/or resolution.

The correlation may be expressed by the correlation function F written in equation (5) above and reproduced below: E=F(S)  Eq(5)

As also indicated above, the correlation function F(S) that gives the value of the primary peak-energy E_(p) measured in the primary pixel 1210 as a function of the negative values S of the induced secondary signal measured on the anode of pixel 1208 is not an analytical function but instead is derived empirically in various embodiments.

FIG. 17 provides a graph 1300 containing example empirical functions F(S) that may be derived from the radiation detector assembly 1200 of FIG. 16A by measuring and correlating the values of the primary signals E shown by graph 1242 (FIG. 16B) and the negative values S of the secondary induced signals shown by graph 1250 (FIG. 16C). For each secondary negative value S measured on the anode of the secondary pixel 1208 (graph 1250), multiple measurements are done for the primary signals E measured on the anode of the primary pixel 1210 (graph 1242). The multiple primary signals E measured for a certain negative value S represent the spectrum of primary pixel 1210. The maximum value of signals E in the spectrum of the primary pixel 1210 measured for a certain negative value S is selected and represents the peak-energy Erin the spectrum of primary pixel 1210 for a certain negative value of secondary induced signal measured by adjacent pixel 1208.

As seen in FIG. 17, the graph 1300 shows three different examples (curve 1302, curve 1304 and curve 1306) of the primary peak-energy E_(P) versus the negative value S of the corresponding secondary induced signal for three different example aspect ratios between the width W of pixels 1206, 1208, 1210 of the radiation detector assembly 1200 and the thickness D of the radiation detector assembly 1200 (e.g., thickness of semiconductor 1202 in FIG. 16A). For curve 1302, W/D=0.2. For curve 1304, W/D=1. For curve 1306, W/D=1. Curves 1302 and 1304 correspond to nonuniform internal electrical fields, while curve 1306 corresponds to a hypothetical situation including a uniform internal electrical field.

As seen in FIG. 17, the maximum measured peak-energy E_(P0)(DOI₀) is derived when S=0. For any other value S<0, the measured EP<E_(P0)(DOI₀). For example, point 1308 on curve 1304 is the primary peak-energy E_(P1308)<E_(P0)(DOI₀) in the spectrum derived for the negative value of the induced signal S<0. To correct primary peak-energy E_(P1308) to bring its value to be equal to E_(P0)(DOI₀), the primary peak-energy E_(P1308) is multiplied in various embodiments by a correction factor K that is given as follows: K=E _(P0)(DOI₀)/E _(P1308)(DOI₁₃₀₈); or  Eq(7) K(S)=E _(P0)(S ₀)/E _(P1308)(S ₁₃₀₈)  Eq(8)

where E_(P0)(S₀) and E_(P1)(S₁₃₀₈) are the peak-energies corresponding to negative values S₀ and S₁₃₀₈. Accordingly, in various embodiments, the energy correction factor may be determined using a plot of the negative value against the value of the primary signal. The value of correction factor K is also equal to the ratio between the lengths of lines 1312 and 1310 in various embodiments. Accordingly, the energy correction factor may be defined by the relationship between a magnitude of the primary value and the value of the negative signal corresponding to the ratio between the peak-energy E_(P0) (S₀) for DOI₀=0 and S₀=0 and the peak energy E_(P1308)(S₁₃₀₈) for DOI₁₃₀₈ and S₁₃₀₈ (e.g., length of line 1312 and length of line 1310). Point 1308 on curve 1306 is used, for example, to demonstrate how the value of correction factor K may be calculated. Such calculations for factor K can be done for correcting any point on curves 1302, 1304, or 1306 for correcting their values of the primary peak-energy E_(P) to make it equal (or approximately equal) to primary peak-energy E_(P0) (DOI₀).

The peak-energy E_(P) of curve 1302 (with the relatively lower aspect ratio of 0.2 compared to the other curves 1304, 1306) depends relatively weakly on the negative value S since the aspect ratio W/D=0.2 results in a relatively strong small-pixel effect that reduces the dependency on the DOI and thus on S relative to the curves corresponding to an aspect ratio of 1.

In contrast, the peak-energy E_(P) of curve 1306 more strongly depends on the negative value S since the aspect ratio is higher (W/D=1) and does not produce a significant small-pixel effect. Accordingly for curve 1306, there is a relatively strong dependency on the DOI and thus on S.

The peak-energy E_(P) of curve 1304 moderately depends (e.g., less strongly than curve 1306 and more strongly than curve 1302) on the negative value S since the aspect ratio is 1, and does not produce a significant small-pixel effect. However, the nonuniform internal electrical field in the detector may produce an effect somewhat similar to the like small-pixel effect (see, e.g., U.S. Pat. No. 9,954,132 entitled “Systems and Methods for Detectors Having Improved Internal Electrical Fields” issued Apr. 24, 2018.) Accordingly, the curve 1304 has a relatively moderate dependency on the DOI and thus on S (e.g., less of a dependency than exhibited by curve 1306).

It may be noted that, alternatively, the correction factor K(S) of equation (8) above may be derived in various embodiments as explained below, with reference to FIG. 10 discussed above. As discussed above, FIG. 10 schematically illustrates a calibration system 92 that may be used to determine a correlation function of negative values S versus the DOI as shown by equation (2), reproduced as follows: S=g(DOI)  Eq(2)

The radiation source 97 of calibration system 92 may scan the detector 93 by irradiating the side-wall of detector 93 at different DOI's to acquire detector spectra for different values of DOI. For each DOI, there is a specific negative value S of the induced secondary signal, which is common to all the events in the spectrum acquired at a certain DOI for a primary pixel. For each DOI, a detector spectrum may be acquired, in a primary pixel, from which the peak-energy E_(P) is derived. Accordingly, for each DOI, a correlation between the peak-energy E_(P) and the negative secondary signal S may be derived. By scanning the detector for various values of DOI, a correlation function of E_(P) versus S may be derived to empirically measure the correlation function F(S) shown in equation (5) (i.e., E=F(S)).

In various embodiments, the correction factor K(S) may be derived by the ratio between the energies received from equation (5) for the values S₀ and S₁₃₀₈ of graph 1300 in FIG. 17 to yield equation (8), reproduced again below: K(S)=E _(P0)(S ₀)/E _(P1308)(S ₁₃₀₈)  Eq(8)

As explained in more detail in connection with the descriptions below associated with FIGS. 18 and 19, applying energy correction factors as discussed herein to the spectrum produced by the radiation detector assembly 1200 improves the energy-resolution by narrowing the peak of the spectrum, and improves the sensitivity by reducing the number of events in the low-energy tail of the spectrum by moving events from the tail into the peak of the spectrum.

FIG. 18 schematically illustrates multiple spectra 1400 of the radiation detector assembly 1200. Spectrum 1402 of radiation detector assembly 1200 shows a histogram of the number of counts N (number of events) versus the energy E of the events. Spectrum 1402 may be understood as a super-positioning of unobserved multiple sub-spectra 1404, 1406, 1408, 1410, with each of the sub-spectra corresponding to a certain and specific negative value S of the secondary induced signal. It may be noted that only part of the multiple sub-spectra that may make up spectrum 1402 are shown in FIG. 18 for clarity and ease of illustration. In the illustrated example, each of the sub-spectra 1404, 1406, 1408, 1410 corresponds to negative specific values S of the induced secondary signals S₀=0, S₁, S₂ and S respectively. (It may be noted that each sub-spectra need not be limited to only a single isolated value or range of S). For example, each sub-spectra may include a relatively narrow range of values corresponding to a single nominal value of S.)

As seen in FIG. 18, the values of the peak-energies E_(P) 1412, 1414 and 1416 marked as E_(P0), E_(P1) and E_(P2) of spectra 1404, 1406 and 1408, respectively, have magnitudes that trend lower with the negative value of S (e.g., the more negative is S the lower is the energy E of the peak-energy E_(P)). As also seen in FIG. 18, the number of counts/events N in spectra 1404, 1406 and 1408, corresponding to S₀=0, S₁ and S₂, respectively, tends to go down with the negative value of S (e.g., the more negative is S the lower is number of N in the spectrum). This is due to the negative value S becoming more negative due to the value of the DOI, and the absorption of the events in the region of the DOI goes down with the value of the DOI. Accordingly, the magnitude of the negative value of S goes up (becomes more negative) with the value of the DOI, but N goes down with the value of the DOI. Accordingly, N goes down with the negative value of S.

The sub-spectra 1404, 1406, 1408, 1410 have different positions of their peak-energies for the different values of S. As such, the sub-spectra are shifted relative to each other. The more negative is the value S corresponding to a given sub-spectrum, the larger is the peak shift toward the lower energies for that given sub-spectra. In the illustrated example, the shift between sub-spectra 1404 and 1406 is relatively small, but contributes to the broadening of the peak of spectrum 1402, and accordingly degrades the energy resolution of radiation detector assembly 1200. As another example, the shift between sub-spectra 1404 and 1408 is relatively large and contributes to the increase in the number of events in the low-energy tail of spectrum 1402, and accordingly degrades the sensitivity of radiation detector assembly 1200. As such, sub-spectra corresponding to small negative values S tend to degrade the energy resolution of the spectrum 1402 of radiation detector assembly 1200 while, sub-spectra corresponding to large negative values S tend to degrade the sensitivity of spectrum 1402 of radiation detector assembly 1200.

Sub-spectra such as spectra 1404, 1406, 1408, 1410 that are not observable conventionally (i.e., by obtaining a histogram of the number of events N versus the energy E of the events), may be revealed and obtained as described above in connection with FIGS. 16A-C. Accordingly, in various embodiments, sub-spectra may be acquired by breaking down spectrum 1402 into its component sub-spectra by dividing the events of spectrum 1402 into multiple groups, with the events in each group corresponding to a similar negative value S, and each one of the groups corresponding to a different negative value S. Each group of these groups represents a sub-spectra corresponding to a certain value S (e.g., a range centered on a particular value) that is different from the other groups. After sub-spectra have been obtained, the correction factors K, derived for example as discussed in connection with FIG. 17, may be applied on the sub-spectra to improve the energy-resolution and sensitivity of radiation detector assembly 1200, for example as shown and discussed in connection with FIG. 19.

FIG. 19 provides a graph 1500 illustrating an example of application of a correction factor K on the energies of the events in sub-spectra for improving the energy-resolution and the sensitivity of radiation detector assembly 1200. It may be noted that the same sub-spectra 1404, 1406, 1408, 1410 of FIG. 18 appear in FIG. 19, but in different energy-positions of their peak-energy. Accordingly, the same reference numerals are used in FIGS. 18 and 19 for the sub-spectra. Similarly, the same reference numeral 1402 is used for the uncorrected spectrum 1402 of FIG. 18 and the corrected spectrum 1402 in FIG. 19. Similarly, peak-energy E₀ is marked 1412 in both figures.

Generally, events in each sub-spectra have corresponding value S that is different than the value for the other sub-spectra. Accordingly, events in each particular sub-spectra may be corrected by a specific correction factor K(S) corresponding to the specific value of S that belongs to that particular sub-spectra. After applying the appropriate correction factors K(S) on each sub-spectra 1404, 1406, 1408, 1410, the sub-spectra keep their shape but all of them are shifted to align their peak-energy with peak-energy E₀. The energy position of peak-energy E₀ is the same, prior and post correction since the correction factor K is equal to 1 for this peak-energy.

As seen in FIG. 19, the super position of the sub-spectra generates spectrum 1402 with peak-energy E₀, improving energy-resolution and sensitivity compared to the uncorrected spectrum 1402 of FIG. 18. For example, corrected spectrum 1402 of FIG. 19 has a peak that is narrower than the peak of the un-corrected spectrum 1402 in FIG. 18 and accordingly has improved energy resolution. Similarly, the corrected spectrum 1402 of FIG. 19 has a lower low-energy tail (or less events in the tail) and accordingly improved sensitivity since the energy correction (energy shift) moves events from the low-energy tail into the peak of the spectrum (or from outside of a window used for counting events to inside a window used for counting events). The peak of spectrum 1402 in FIG. 19 includes more events than the peak of spectrum 1402 in FIG. 18 since in FIG. 19 all the peaks of the sub-spectra are aligned to the same peak energy 1412 (E₀=Ep₀) This alignment also causes to the peak of spectrum 1402 in FIG. 19 to appear at higher peak-energy than the peak-energy of spectrum 1402 in FIG. 18.

It may be noted that general principles discussed herein may be applied on a sub-pixel basis. For example, in various embodiments, the processing unit 120 is configured to determine energy correction factors for sub-pixels of the pixelated anodes, and to apply the energy correction factors on a sub-pixel basis. Additionaly details regarding the use of sub-pixels may be found in U.S. Pat. No. 9,927,539 (“Systems and Methods for Improving Imaging by Sub-Pixel Calibration”), referred to herein as the 539 Patent, the entire subject matter of which is hereby incorporated.

It may be noted that the calibration and/or correction techniques discussed herein to improve energy resolution and/or reduce the low energy tail may also be useful for sub-pixels as well. For example, various calibration techniques disclosure herein may be applied based on sub-pixels containing the events of the primary pixels, with locations in X and Y directions inside the primary pixels derived using the secondary signals. While the techniques discussed herein generally relate to energy calibration and/or correction for different DOI's or along the Z direction, the techniques disclosed in the 539 patent relate to energy calibration and/or correction along the lateral or X and Y directions (e.g., to account for in homogeneities with the pixels along the lateral directions).

Accordingly, in various embodiments, techniques from the 539 patent may be used to correct energies in sub-pixels along the lateral directions, and techniques disclosed herein may be used to correct energies in sub-pixels along the vertical or Z direction. The combination of techniques may be used to yield three dimensional energy corrections for sub-pixels. For example, the negative value S and the values of the inner coordinates X,Y of the appropriate sub-pixel may be assigned to each event. It may be noted that the corrections may be performed in the vertical direction first and subsequently for the lateral directions, or vice versa.

In various embodiments, the corresponding values of S, X, and Y are assigned for each event. Accordingly, each event may be understood as being tagged by the values of the parameters (S, X, Y). Correction may then be applied as follows. All the events may be divided into different groups such that each group includes events with different values (S, X, Y), and the events within the same group includes events with similar (S, X, Y) (e.g., events for a certain group are included within relatively narrow ranges of S, X, and Y, respectively). Next, a spectrum is produced for each group of events, and the energy-peak of each group of events is derived. Next, the energy-peak with the maximal energy value out of all the energy values of the energy-peaks of the different spectra of the different groups are derived and defined as the maximum energy-peak. The calibration factor K for each group of events may then be defined as the ratio between the energy value of the maximum energy-peak and the energy value of the energy-peak of the group.

FIG. 20 provides a flowchart of a method 2000 (e.g., for determining an energy correction factor as discussed herein), in accordance with various embodiments. The method 2000, for example, may employ or be performed by structures or aspects of various embodiments (e.g., systems and/or methods and/or process flows) discussed herein. In various embodiments, certain steps may be omitted or added, certain steps may be combined, certain steps may be performed concurrently, certain steps may be split into multiple steps, certain steps may be performed in a different order, or certain steps or series of steps may be re-performed in an iterative fashion. In various embodiments, portions, aspects, and/or variations of the method 2000 may be able to be used as one or more algorithms to direct hardware (e.g., one or more aspects of the processing unit 120 and/or processing unit 1290) to perform one or more operations described herein.

At 2002, primary signals and secondary signals are acquired corresponding to acquisition events (e.g., events corresponding to reception of photons). The primary and secondary signals are generated responsive to reception of photons by a semiconductor detector, and are received from pixelated anodes (e.g., anodes of a semiconductor device of an imaging system such as assembly 100 or assembly 1200). For example, a patient that has been administered at least one radiopharmaceutical may be placed within a field of view of one or more detectors, and radiation (e.g., photons) emitted from the patient may impact the pixelated anodes disposed on reception surfaces of the one or more detectors resulting in acquisition events (e.g., photon impacts). For a given photon impact in the depicted example embodiment, a primary signal (responsive to a collected charge) is generated by the impacted pixelated anode (or collecting anode), and one or more secondary signals (responsive to non-collected charge) are generated by pixelated anodes adjacent to the impacted pixelated anode (or non-collecting anodes). The secondary signals defining corresponding negative values as discussed herein.

At 2004, an energy correction factor for the semiconductor device is determined for acquisition events resulting in the primary and secondary signals acquired at 2002. In various embodiments, the energy correction factor for a given event is determined using at least one secondary signal for that particular event. For example, as discussed herein, the energy correction factor in various embodiments is determined using a negative value for at least one secondary signal from one or more adjacent or non-collecting pixels (e.g., at least one adjacent pixelated anode). For example, the energy correction factor for each event may be identified based on the negative value of a secondary signal for that event. It may be noted that the energy correction factor in various embodiments is determined without using any information (e.g., detected charge or corresponding signals) from a cathode of the detector assembly.

In various embodiments, the energy correction factors may be determined using calibration information. For example, in the depicted embodiment, at 2006, calibration information is used to determine the energy correction factor. For example, for each event, an appropriate energy correction factor may be identified based on the negative value of the secondary signal for the event. In various embodiments, the energy correction factors for different negative values may be predetermined as part of a calibration process. For example, the energy correction factor for a given event may be identified using a calibration based on a ratio between a negative value of at least one secondary signal for the event and a corresponding amplitude or magnitude of the primary signal.

At 2008, a sub-pixel location is determined using the primary signal and the at least one secondary signal. For each event, a corresponding sub-pixel location may be determined. It may be noted that the same information (primary and secondary signals) used to determine energy correction factors for events may also be used to determine sub-pixel locations for those events.

At 2010, an energy level for an event is adjusted using the appropriate energy correction factor. In various embodiments, as discussed herein, the energy correction factor is selected using the negative value of a secondary signal for the event. The energy levels are adjusted in various embodiments to make the energy levels for a group of events more consistent and/or closer to a target (e.g., a maximal peak energy).

At 2012, an image is reconstructed using the adjusted energy levels of the events (e.g., using counts of events within a predetermined window of energy levels after adjusting the energy levels).

As discussed herein, a radiation detector system may be calibrated to provide predetermined energy correction factors based on negative values of secondary signal for events. FIG. 21 provides a flowchart of a method 2100 (e.g., for providing and calibrating a radiation detector assembly), in accordance with various embodiments. The method 2100, for example, may employ or be performed by structures or aspects of various embodiments (e.g., systems and/or methods and/or process flows) discussed herein. In various embodiments, certain steps may be omitted or added, certain steps may be combined, certain steps may be performed concurrently, certain steps may be split into multiple steps, certain steps may be performed in a different order, or certain steps or series of steps may be re-performed in an iterative fashion. In various embodiments, portions, aspects, and/or variations of the method 2100 may be able to be used as one or more algorithms to direct hardware (e.g., one or more aspects of the processing unit 120 and/or processing unit 1290) to perform one or more operations described herein.

At 2102, a semiconductor detector (e.g., semiconductor 1202 of radiation detector assembly 1200) is provided. The semiconductor detector of the illustrated example has a surface with plural pixelated anodes disposed on the surface. Each pixelated anode is configured to generate a primary signal responsive to reception of a photon by the pixelated anode and to generate at least one secondary signal responsive to an induced charge caused by reception of a photon by at least one adjacent anode. At 2104, the pixelated anodes are operably coupled to at least one processor (e.g., processing unit 1290).

At 2106, a calibrated radiation supply (e.g., having a known photon energy) is provided. In some embodiments, the calibrated radiation supply is provided at different depths along a sidewall of the semiconductor detector. Responsive to reception of the calibrated radiation supply, the pixelated anodes generate primary and secondary signals. For example, the calibrated radiation supply may be passed through a pin-hole collimator to the sidewall of the semiconductor detector. The location of a given pin-hole (e.g., in a Z direction) through which radiation is passed may be used to determine the DOI at which the corresponding radiation is passed from the collimator and received by the semiconductor detector, and events may be grouped by DOI. Alternatively, events may be generated at unknown depths and grouped based on a negative value generated by a secondary signal for the event. At 2108, the primary and secondary signals are acquired from the pixelated anodes by the at least one processor.

At 2110, corresponding negative values of total induced charges (e.g., corresponding to the various different depths at which the radiation has been absorbed) are determined. At 2112, calibration information (e.g., a look-up table or other correlating relationship between negative induced non-collected charge values and energy correction factors) is determined. For example, for events having a given negative value X (e.g., negative values within a predetermined range of X), relationships between the magnitude of the primary signal and the magnitude of the negative value may be used to define a correction factor that may be used to adjust the energy levels of events having the given negative value X.

It should be noted that the various embodiments may be implemented in hardware, software or a combination thereof. The various embodiments and/or components, for example, the modules, or components and controllers therein, also may be implemented as part of one or more computers or processors. The computer or processor may include a computing device, an input device, a display unit and an interface, for example, for accessing the Internet. The computer or processor may include a microprocessor. The microprocessor may be connected to a communication bus. The computer or processor may also include a memory. The memory may include Random Access Memory (RAM) and Read Only Memory (ROM). The computer or processor further may include a storage device, which may be a hard disk drive or a removable storage drive such as a solid-state drive, optical disk drive, and the like. The storage device may also be other similar means for loading computer programs or other instructions into the computer or processor.

As used herein, the term “computer” or “module” may include any processor-based or microprocessor-based system including systems using microcontrollers, reduced instruction set computers (RISC), ASICs, logic circuits, and any other circuit or processor capable of executing the functions described herein. The above examples are exemplary only, and are thus not intended to limit in any way the definition and/or meaning of the term “computer”.

The computer or processor executes a set of instructions that are stored in one or more storage elements, in order to process input data. The storage elements may also store data or other information as desired or needed. The storage element may be in the form of an information source or a physical memory element within a processing machine.

The set of instructions may include various commands that instruct the computer or processor as a processing machine to perform specific operations such as the methods and processes of the various embodiments. The set of instructions may be in the form of a software program. The software may be in various forms such as system software or application software and which may be embodied as a tangible and non-transitory computer readable medium. Further, the software may be in the form of a collection of separate programs or modules, a program module within a larger program or a portion of a program module. The software also may include modular programming in the form of object-oriented programming. The processing of input data by the processing machine may be in response to operator commands, or in response to results of previous processing, or in response to a request made by another processing machine.

As used herein, a structure, limitation, or element that is “configured to” perform a task or operation is particularly structurally formed, constructed, or adapted in a manner corresponding to the task or operation. For purposes of clarity and the avoidance of doubt, an object that is merely capable of being modified to perform the task or operation is not “configured to” perform the task or operation as used herein. Instead, the use of “configured to” as used herein denotes structural adaptations or characteristics, and denotes structural requirements of any structure, limitation, or element that is described as being “configured to” perform the task or operation. For example, a processing unit, processor, or computer that is “configured to” perform a task or operation may be understood as being particularly structured to perform the task or operation (e.g., having one or more programs or instructions stored thereon or used in conjunction therewith tailored or intended to perform the task or operation, and/or having an arrangement of processing circuitry tailored or intended to perform the task or operation). For the purposes of clarity and the avoidance of doubt, a general purpose computer (which may become “configured to” perform the task or operation if appropriately programmed) is not “configured to” perform a task or operation unless or until specifically programmed or structurally modified to perform the task or operation.

As used herein, the terms “software” and “firmware” are interchangeable, and include any computer program stored in memory for execution by a computer, including RAM memory, ROM memory, EPROM memory, EEPROM memory, and non-volatile RAM (NVRAM) memory. The above memory types are exemplary only, and are thus not limiting as to the types of memory usable for storage of a computer program.

It is to be understood that the above description is intended to be illustrative, and not restrictive. For example, the above-described embodiments (and/or aspects thereof) may be used in combination with each other. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the various embodiments without departing from their scope. While the dimensions and types of materials described herein are intended to define the parameters of the various embodiments, they are by no means limiting and are merely exemplary. Many other embodiments will be apparent to those of skill in the art upon reviewing the above description. The scope of the various embodiments should, therefore, be determined with reference to the appended claims, along with the full scope of equivalents to which such claims are entitled. In the appended claims, the terms “including” and “in which” are used as the plain-English equivalents of the respective terms “comprising” and “wherein.” Moreover, in the following claims, the terms “first,” “second,” and “third,” etc. are used merely as labels, and are not intended to impose numerical requirements on their objects. Further, the limitations of the following claims are not written in means-plus-function format and are not intended to be interpreted based on 35 U.S.C. § 112(f) unless and until such claim limitations expressly use the phrase “means for” followed by a statement of function void of further structure.

This written description uses examples to disclose the various embodiments, including the best mode, and also to enable any person skilled in the art to practice the various embodiments, including making and using any devices or systems and performing any incorporated methods. The patentable scope of the various embodiments is defined by the claims, and may include other examples that occur to those skilled in the art. Such other examples are intended to be within the scope of the claims if the examples have structural elements that do not differ from the literal language of the claims, or the examples include equivalent structural elements with insubstantial differences from the literal language of the claims. 

What is claimed is:
 1. A radiation detector assembly comprising: a semiconductor detector having a surface; plural pixelated anodes disposed on the surface, each pixelated anode configured to generate a primary signal responsive to reception of a photon by the pixelated anode and to generate at least one secondary signal responsive to an induced charge caused by reception of a photon by at least one surrounding anode; and at least one processor operably coupled to the pixelated anodes, the at least one processor configured to: acquire a primary signal from one of the anodes responsive to reception of a photon by the one of the anodes; acquire secondary signals from neighboring pixels of the one of the anodes responsive to an induced charge caused by the reception of the photon by the one of the anodes, the secondary signals defining corresponding negative values; organize detected events into groups based on magnitudes of the corresponding negative values; and determine an energy correction factor for the reception of the photon by the one of the anodes using the groups.
 2. The radiation detector assembly of claim 1, wherein the at least one processor is configured to adjust an energy level of detected events using the energy correction factor.
 3. The radiation detector assembly of claim 1, wherein the at least one processor is configured to use a relationship of a value of the primary signal with the negative value of at least one secondary signal to determine the energy correction factor.
 4. The radiation detector assembly of claim 3, wherein the at least one processor is configured to determine the energy correction factor using a plot of the negative value against the value of the primary signal.
 5. The radiation detector assembly of claim 1, wherein the energy correction factor is defined by a relationship between a magnitude of the primary signal and a magnitude of at least one of the negative values.
 6. The radiation detector assembly of claim 1, wherein the at least one processor is configured to apply the energy correction factor to move at least one detected event from outside of an energy window used for counting to inside the energy window used for counting.
 7. The radiation detector assembly of claim 1, wherein the at least one processor is configured to apply corresponding energy correction factors to the groups, wherein at least one group has a correction factor different from the other groups.
 8. The radiation detector assembly of claim 1, wherein the at least one processor is configured to define sub-spectra based on magnitudes of corresponding negative values for detected events, and align peaks of the sub-spectra using corresponding energy correction factors.
 9. The radiation detector assembly of claim 1, wherein the at least one processor is configured to determine energy correction factors for sub-pixels of the pixelated anodes, and to apply the energy correction factors on a sub-pixel basis.
 10. A method of imaging using a semiconductor detector having a surface with plural pixelated anodes disposed thereon, wherein each pixelated anode is configured to generate a primary signal responsive to reception of a photon by the pixelated anode and to generate at least one secondary signal responsive to an induced charge caused by reception of a photon by at least one surrounding anode, the method comprising: acquiring a primary signal from one of the anodes responsive to reception of a photon by the one of the anodes; acquiring secondary signals from neighboring pixels of the one of the anodes responsive to an induced charge caused by the reception of the photon by the one of the anodes, the secondary signals defining corresponding negative values; organizing detected events into groups based on magnitudes of the corresponding negative values; and determining an energy correction factor for the reception of the photon by the one of the anodes using groups.
 11. The method of claim 10, further comprising adjusting an energy level for an event corresponding to the reception of the photon by the one of the anodes using the energy correction factor.
 12. The method of claim 10, further comprising reconstructing an image using the adjusted energy level.
 13. The method of claim 10, further comprising using calibration information to determine the energy correction factor.
 14. The method of claim 13, further comprising determining the energy correction factor using a calibration based on a relationship between a magnitude of the primary signal and the negative value of at least one secondary signal.
 15. The method of claim 10, further comprising determining the energy correction factor without using any information from a cathode of the detector assembly.
 16. The method of claim 10, further comprising determining a sub-pixel location using the primary signal and at least one secondary signal.
 17. The method of claim 10, further comprising using a relationship of a value of the primary signal with the negative value of the at least one secondary signal to determine the energy correction factor.
 18. The method of claim 10, further comprising applying corresponding energy correction factors to the groups, wherein at least one group has a correction factor different from the other groups.
 19. A method of providing a radiation detector assembly comprising: providing a semiconductor detector having a surface with plural pixelated anodes disposed thereon, each pixelated anode configured to generate a primary signal responsive to reception of a photon by the pixelated anode and to generate at least one secondary signal responsive to an induced charge caused by reception of a photon by at least one adjacent anode; operably coupling the pixelated anodes to at least one processor; providing a calibrated radiation supply to the semiconductor detector, wherein the pixelated anodes generate primary signals and secondary signals responsive to the calibrated radiation supply; acquiring, with the at least one processor, the primary signals and the secondary signals from the pixelated anodes; determining corresponding negative values of total induced charges for the secondary signals corresponding to different depths of absorption; organizing detected events into groups based on magnitudes of the corresponding negative values; determining calibration information based on the groups.
 20. The method of claim 19, wherein the calibrated radiation supply is provided along a sidewall of the semiconductor detector at different depths.
 21. The method of claim 19, wherein the calibration information includes energy correction factors based on corresponding relationships between a magnitude of the primary signal and negative values of the secondary signals. 